Positron emission

In 1934 Frédéric and Irène Joliot-Curie bombarded aluminium with alpha particles (emitted by polonium) to effect the nuclear reaction ⁴
₂He
 + ²⁷
₁₃Al
 → ³⁰
₁₅P
 + ¹
₀n
, and observed that the product isotope ³⁰
₁₅P
emits a positron identical to those found in cosmic rays by Carl David Anderson in 1932.^[2] This was the first example of
β⁺
 decay (positron emission). The Curies termed the phenomenon "artificial radioactivity", because ³⁰
₁₅P
is a short-lived nuclide which does not exist in nature. The discovery of artificial radioactivity would be cited when the husband-and-wife team won the Nobel Prize.

Isotopes which undergo this decay and thereby emit positrons include, but are not limited to: carbon-11, nitrogen-13, oxygen-15, fluorine-18, copper-64, gallium-68, bromine-78, rubidium-82, yttrium-86, zirconium-89,^[3] sodium-22, aluminium-26, potassium-40, strontium-83, and iodine-124.^[3][4] As an example, the following equation describes the beta plus decay of carbon-11 to boron-11, emitting a positron and a neutrino:

11 6C

→

11 5B

+

e+

+

ν e

+

0.96 MeV

Inside protons and neutrons, there are fundamental particles called quarks. The two most common types of quarks are up quarks, which have a charge of +2⁄3, and down quarks, with a −1⁄3 charge. Quarks arrange themselves in sets of three such that they make protons and neutrons. In a proton, whose charge is +1, there are two up quarks and one down quark (2⁄3 + 2⁄3 − 1⁄3 = 1). Neutrons, with no charge, have one up quark and two down quarks (2⁄3 − 1⁄3 − 1⁄3 = 0). Via the weak interaction, quarks can change flavor from down to up, resulting in electron emission. Positron emission happens when an up quark changes into a down quark, effectively converting a proton to a neutron.^[5]

Nuclei which decay by positron emission may also decay by electron capture. For low-energy decays, electron capture is energetically favored by 2m_ec² = 1.022 MeV, since the final state has an electron removed rather than a positron added. As the energy of the decay goes up, so does the branching fraction of positron emission. However, if the energy difference is less than 2m_ec², then positron emission cannot occur and electron capture is the sole decay mode. Certain otherwise electron-capturing isotopes (for instance, ⁷
Be
) are stable in galactic cosmic rays, because the electrons are stripped away and the decay energy is too small for positron emission.

A positron is ejected from the parent nucleus, but the daughter (Z−1) atom still has Z atomic electrons from the parent, i.e. the daughter is a negative ion (at least immediately after the positron emission). Since tables of masses are for atomic masses, ${\displaystyle _{Z}^{A}{\textrm {X}}\rightarrow _{Z-1}^{A}{\textrm {Y}}+_{+1}^{0}{\textrm {e}}^{+}+_{-1}^{0}{\textrm {e}}^{-}}$  , and, since the mass of the positron is identical to that of the electron, the overall result is that the mass-energy of two electrons is required, and the β⁺ decay is energetically possible if and only if the mass of the parent atom exceeds the mass of the daughter atom by at least two electron masses (2m_e c² = 1.022 MeV).^[6]

Isotopes which increase in mass under the conversion of a proton to a neutron, or which decrease in mass by less than 2m_e, cannot spontaneously decay by positron emission.^[6]

These isotopes are used in positron emission tomography, a technique used for medical imaging. The energy emitted depends on the isotope that is decaying; the figure of 0.96 MeV applies only to the decay of carbon-11.

The short-lived positron emitting isotopes ¹¹C (T_1⁄2 = 20.4 min), ¹³N (T_1⁄2 = 10 min), ¹⁵O (T_1⁄2 = 2 min), and ¹⁸F (T_1⁄2 = 110 min) used for positron emission tomography are typically produced by proton irradiation of natural or enriched targets.^[7][8]

•   Live Chart of Nuclides: nuclear structure and decay data (main decay modes) - IAEA